Low Energy Neutron Detection Efficiency Of Lithium Glass And Boronloaded Liquid Scintillation Detectors

Author : Albert Foster
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (111 download)

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Download or read book Polyvinyl Toluene and Lithium Glass Composites for Neutron Detection written by Albert Foster and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Neutron detectors are required for effective detection of special nuclear materials (SNM). In the past, gaseous neutron detectors, such as He-3, were utilized as the primary means of SNM detection. The expansive deployment of the detectors for Radiation Portal Monitors (RPM) has put a strain on the He-3 stockpile. Since the start of the He-3 shortage, there has been a renewed effort by researchers to identify or fabricate novel alternates to combat this pressing issue. Composite detectors offer a promising avenue to address this crisis and have the potential for added functionalization to detect a wide range of radiation types and energies.In this dissertation, two types of composite detectors are fabricated using polyvinyl toluene as the matrix and Li-glass fillers in the form of rods and shards. Both materials are scintillators, which means light is emitted after radiation interactions. The polymer has a shorter decay time and sensitivity to gammas and fast neutrons, while the glass has a longer decay time and is sensitive to thermal neutrons. The coupling of the different decay times allow for discrimination between gammas, thermal neutrons, and fast neutrons which is necessary for neutron detection. Additionally, the hydrogenous polymer matrix thermalizes the incident neutrons to realize a wider range of neutron energy. When two materials are coupled in a single detector, challenges arise due to the dissimilar material properties. The mismatched optical properties result in increased light scattering as the scintillated light propagates through the composite. This lowers the optical transmission of the composite, which could be detrimental to detection efficiency. Three main objectives are considered to address the realization of composite detectors and the effect of light scattering due to added material interfaces. First, a fabrication process is developed for the rods and shards-based composite designs. The composites are then characterized using optical and nuclear techniques. The effect of interfacial surface area on detector performance is quantified experimentally, along with the role that glass volume content plays on light output of the detectors. Second, the role of dopants on the matrix properties is examined using thermal, mechanical and optical characterization techniques. The higher the dopant concentration, the more sensitive the polymer is to fast neutrons. The fabricated detectors in the first objectives only have sensitivities to thermal neutrons and gammas. This highly doped polymer is then used as the matrix to realize triple-pulse-shape discrimination capable composites. Third, an optical light simulation software is utilized to better understand the effect of interfacial surface area of light on light transport. The transmission is modeled for composites with varying glass contents, sizes, shapes, and composite thicknesses.In this study, all three objectives were addressed and studied to achieve a better understanding of using polyvinyl toluene and Li-glass composites for neutron detection. A facile and scalable fabrication method for rods and shards detector was developed. Additionally composites with high interfacial surface area were found to detrimentally affect both the light transmission and the figure of merit of the composite. These results informed the design of the triple-pulse-shape discrimination capable composite detector. The dopants used to realize fast neutron sensitivity in the polymer matrix were found to act as plasticizers, creating a mechanical soft material at 25C. Crosslinking molecules were added to mitigate the plasticizer effect, and storage modulus of matrix with the same dopant concentration increased by an order of magnitude. Therefore a more structurally sound polymer was synthesized without a decrease in detection performance. Finally, the optical light transport model was successfully validated which allowed for accurate simulation of transmission through the composite. The goal of the model was to better understand the relationship of the glass filler parameters, such as interfacial surface area, glass content, and detector thickness, with the propagation of optical light. The model could eventually be used for predicting the transmission through a theoretical composite without the need of a physical sample. It could also be used to guide the experimental fabrication of the composite to maximize nuclear and optical performance. Findings from the model confirmed that interfacial surface area and glass content had an inverse relationship on the composites light transmission in a roughly linear trend. The thickness of the composite also had an inverse relationship, and followed an exponential decay.The important findings of this work to the neutron detection community are wide ranging. First we have identified that filler size and shape have a measureable effect on the gamma rejection and light transmission, which informs future composite detector design. Second, the PSD capable crosslinked PVT resin offers an avenue for the fabrication of a scalable triple PSD composite detector. Third, the ability to model light transport through a composite provides researchers with a tool to simulate optical light transmission without fabricating a physical detector. The model can accurately simulate the light transmission through a composite detector for a wide range of detection parameter such as filler size, shape, content, and composite thickness. Additionally this model can be used in conjunction with nuclear simulations to quantify effect of filler parameters on detection performance.